A digital subscriber line (DSL) technology is a high-speed transmission technology for performing data transmission by using a telephone twisted pair, that is, an unshielded twisted pair (UTP), and includes asymmetric digital subscriber line (ADSL), very-high-bit-rate digital subscriber line (VDSL), integrated services digital network (ISDN) digital subscriber line (IDSL), and the like. These digital subscriber lines are collectively referred to as xDSL.
Except baseband transmission DSLs such as IDSL and symmetric high-bit-rate digital subscriber line (SHDSL), a passband transmission xDSL enables, by using a frequency division multiplexing technology, an xDSL service to coexist on the same twisted pair as a plain old telephone service (POTS), where the xDSL service occupies a high frequency band, the POTS occupies a baseband part below 4 kilohertz (KHz), and a POTS signal and an xDSL signal are split by using a splitter. The passband transmission xDSL uses discrete multi-tone (DMT) modulation. A system providing access to multiple xDSLs is referred to as a digital subscriber line access multiplexer (DSLAM). According to the electromagnetic induction principle, mutual interference is caused among multiple channels of signals received by the DSLAM, which is referred to as crosstalk and includes near end crosstalk (NEXT) and far end crosstalk (FEXT). NEXT and FEXT energy increases as a frequency band increases. On one hand, because frequency division multiplexing is used for xDSL upstream and downstream channels, NEXT does not cause serious harm on system performance. On the other hand, because a frequency band used by the xDSL becomes wider, FEXT affects transmission performance of a line more seriously. According to Shannon's equation C=B·log2 (1+S/N) (where C is a channel capacity, B is a signal bandwidth, S is signal energy, and N is noise energy), because crosstalk in an xDSL reflects a part of noises, serious FEXT significantly reduces a channel rate. When an xDSL service is provisioned in a bundle of cables upon the requests of multiple subscribers, FEXT may cause a low rate, unstable performance, and even a failure in service provisioning for some lines, which eventually results in a low line activation ratio of the DSLAM.
Currently, a technology known as a vectored digital subscriber line (Vectored-DSL) is proposed in the industry. The Vectored-DSL mainly uses a possibility of joint sending and receiving on a DSLAM end to cancel FEXT interference by using a signal processing method, thereby eventually eliminating FEXT interference in each channel of signals. As shown in FIG. 1a and FIG. 1b which respectively provide a schematic diagram of synchronous sending and synchronous receiving on a DSLAM end, an operating principle thereof is described as follows:
In FIG. 1a and FIG. 1b, a shared channel H on a kth tone of a frequency domain may be expressed in a matrix form:
                    H        =                              [                                                                                h                    11                                                                                        h                    12                                                                    …                                                                      h                                          1                      ⁢                      M                                                                                                                                        h                    21                                                                                        h                    22                                                                    …                                                                      h                                          2                      ⁢                      M                                                                                                                                                                                                                                                                                                                                                                                      ⁢                                          h                      ij                                                                                                                                                                                                            ⋮                                                  ⋮                                                  ⋱                                                  ⋮                                                                                                  h                                          M                      ⁢                                                                                          ⁢                      1                                                                                                            h                                          M                      ⁢                                                                                          ⁢                      2                                                                                        …                                                                      h                    MM                                                                        ]                                M            ×            M                                              (                  equation          ⁢                                          ⁢          1                )            
In equation 1, hij is a transmission equation from a line pair j to a line pair i. Actually, i equals j and equals the number of channels having a crosstalk relationship with each other in a shared channel. On an assumption that the number of channels having a crosstalk relationship with each other in a shared channel is M, H is an M×M channel transmission matrix. Moreover, it is respectively assumed that x is an M×1 channel input vector, y is an M×1 channel output vector, and n is an M×1 noise vector. Eventually, the channel transmission equation is expressed in the following form:y=Hx+n  (equation 1)
In an upstream direction, joint receiving processing on a signal is performed on a central office (CO) end. That is, a crosstalk canceller (expressed by using a matrix W) is added to a vectored digital subscriber line access multiplexer (Vectored DSLAM; refer to FIG. 1c for functional modules thereof) on the CO end, and therefore a signal received by the CO end is:{tilde over (y)}=Wy=WHx+Wn  (equation 2)
When a matrix WH is a diagonal matrix, the crosstalk is eliminated.
In a downstream direction, joint sending processing on a signal is performed on the CO end. That is, a crosstalk pre-coder (expressed by using a matrix P) is added to the Vectored DSLAM on the CO end, and therefore a signal sent by the CO end is:{tilde over (x)}=Px  (equation 3)
A signal received by a receiving end (subscriber end) is:{tilde over (y)}=H{tilde over (x)}+n=HPx+n  (equation 4)
When a matrix HP is a diagonal matrix, the crosstalk is also eliminated.
According to the above analysis, a Vectored-DSL performs joint processing in the upstream and downstream directions to achieve an effect of eliminating far end crosstalk, where a key point thereof is to estimate a downstream precoding matrix P and an upstream cancellation matrix W.
Generally, a Vectored-DSL system is implemented by using the following method. Firstly, synchronization is performed using a synchronization symbol (Sync Symbol. Then, joint modulation is performed on a pilot sequence on Sync Symbols of all lines. Finally, a receiving side feeds back an error to a vectoring control entity (VCE). Hence, a downstream precoding matrix P and an upstream cancellation matrix W may be estimated in the VCE, thereby canceling FEXT by using the vectoring technology described above.
A VDSL2 technology is prior to the Vectored-DSL technology and has been widely applied; therefore, compatibility with an existing VDSL2 legacy (Legacy) customer premises equipment (CPE), that is, a VDSL2 Legacy CPE, on an existing network must be considered when upgrading a VDSL2 to a Vectored-DSL. However, the VDSL2 Legacy CPE does not support pilot sequence sending and receiving or error feedback on a synchronization symbol.
In order to support pilot sequence sending and receiving and error feedback on a synchronization symbol (Sync Symbol) to estimate a downstream precoding matrix P and an upstream cancellation matrix W, a method provided by the prior art is to upgrade all VDSL2 Legacy CPEs on an existing VDSL2 network to or replace them with VDSL2 Vectored CPEs.